The composition of a bourbon whiskey barrel. Diagrams showing a stylized bourbon whiskey barrel (scale bar: 15 cm), a cross section of the charred barrel interior and a stave. The sampling scheme highlights the charred surface of the barrel stave (C); the inner portion of wood, which undergoes thermal degradation and distillate penetration (P); the red line (R), which indicates the depth to which the distillate penetrated the stave; and the outer portion of the stave (O). Note that the thickness of the red line and its distance from the outside of the stave vary both within the stave and among staves.